let plugin = {
    extractReducers: [
        {
            key1: (state, action) => state
        },
        {
            key2: (state, action) => state
        }
    ]
}
let reducers = Object.assign({}, plugin.extractReducers[0], plugin.extractReducers[1]);
console.log(reducers);

let rootReducer = combineReducers({ ...reducers });

function combineReducers(reducers) {

    return function (state, action) {
        let newState = {};
        newState['key1'] = { number: 1 };
        newState['key2'] = { number: 1 };
        return newState;
    }
}